Gaithersburg High School cheerleaders welcome students back to school on Monday. (Astrid Riecken /The Washington Post)

Maybe they’re showing off their new Hello Kitty or Spider-Man backpacks. Maybe they made a friend on the bus. Or maybe they cried their eyes out before the first bell rings, terrified of saying goodbye to Mom and Dad.

Or, maybe Mom and Dad were the ones to cry.

Monday marked the first day of school for more than 200,000 public school students in Montgomery County and the District.

This is the 30th year of growth for Montgomery County’s school system and the first time enrollment has exceeded 150,000, with an estimated 2,500 new students expected to file into classrooms Monday.

To accommodate the swelling number of students, Montgomery opens four modernized and expanded campuses Monday: Glenallan Elementary School, Weller Road Elementary School, Herbert Hoover Middle School and Gaithersburg High School.

Along with the additional students, Montgomery is welcoming 800 new teachers to the district and dozens of new principals.

Montgomery school employees were up even before the sun as Superintendent Joshua P. Starr greeted bus drivers at the depot as early as 5 a.m. before they set off on their routes.

The District’s school system, meanwhile, is fighting to maintain enrollment as it faces increasing competition from public charter schools.

More than 45,000 D.C. students headed back to class Monday, many to newly modernized buildings, including Dunbar High — where officials recently cut the ribbon on a new $122 million campus— and Cardozo Education Campus, where the crumbling building was gutted and renovated for $130 million.

Many D.C. charter schools also began Monday, including two that are opening their doors for the first time: Somerset Prep in Southeast Washington and Sela, a Hebrew-immersion school in Ward 4.

A third new charter, Ingenuity Prep, opened earlier in August, while a fourth — Community College Preparatory Academy — opens in mid-September.

Many other charters are expanding to new campuses or additional grade levels. In 2012-13, charters enrolled 43 percent of the city’s public school students.

At Cardozo’s made-over campus, volunteers offered an energetic welcome early Monday, chanting as students made their way up the front steps, down a red carpet and through an arch of purple and white balloons.

Tenth-grader Sheila Paredes hung back with her mother, taking it all in. “I’m nervous,” said the teenager, who is new to Cardozo after moving to the District from Maryland.

Cardozo expanded this year to serve middle school students as well as high school students, taking in children who would have attended Shaw Middle, which closed in June.

Sixth-grader Angelica Edwards said she was excited about having a locker of her own. Her mother, Monique Edwards-Kellam, said she is thrilled by the new building and the promise of a rich curriculum.

“It’s an excellent start to a bright future for these kids,” she said.